Lindholm: What counts as a pitch count?

From Scott Lindholm at Beyond the Box Score on March 13, 2014:

With the emphasis placed on pitch count it's surprising to find no consistent definition of what it is. In addition, there isn't much value given to career pitches thrown, or what constitutes a career pitch. When does the count begin--Little League ball, high school, college, minors? Does it matter only to starters? Do playoff pitches count, as well as spring training? Are days throwing when the pitcher isn't in the game taken into account? In other words, if pitch count serves as some manner of clock, when does the clock start marking time?

Accurate pitch data is available at Baseball-Reference.com going back to 1988, so I looked for pitchers whose careers began in 1988 or later, eliminating players like Roger Clemens and Greg Maddux who threw over 50,000 pitches but had careers beginning before 1988. It includes current pitchers like Madison Bumgarner and Jordan Zimmermann who are still early enough in their careers to have not thrown a large number of pitches.